The present invention relates generally magnetic resonance imaging (MRI), and more particularly, to a method and apparatus, including a new pulse sequence, to achieve greater sensitivity in detecting infarcted myocardial tissue.
MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in human tissue, is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B1) applied at a frequency equal to the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be nutated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated (as the excited spins decays to the ground state) and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Myocardial infarction is a type of cardiac syndrome in which oxygen is deprived from a portion of the heart. The size of the myocardial infarct has been demonstrated to have a strong correlation with patient outcome/recovery. Myocardial perfusion imaging is a technique in which regions of abnormal or impaired blood flow to the heart are detected by tracking the passage of a tracer or contrast agent through myocardial tissue. Regions of impaired blood flow or poor perfusion would not exhibit the presence of the contrast material or tracer whereas in tissues with normal perfusion, the presence of the contrast material or tracer would be indicated.
The imaging of tissue (blood) perfusion is closely related to the imaging of blood flow in vascular structures, such as in MR angiography (MRA). As with MRA, MR perfusion imaging is performed by injecting a volume a contrast agent, such as gadolinium chelate, into the blood stream, conventionally via an intravenous injection. The volume or mass of contrast agent administered is typically referred to as a bolus as it is delivered in a tight volume at a relatively high volume delivery rate (usually 1-5 ml/sec). Differing agents can either decrease the T1 of blood to enhance the detected MR signal, or decrease the T2 of blood to attenuate the detected MR signal. As the bolus passes through the body, the enhanced or attenuated signal increases or decreases the signal intensity observed in perfused tissue, but not in the non-perfused tissue. The degree of signal change in the observed tissue as compared with baseline images acquired prior to the arrival of the contrast material can be used to determine the degree of tissue perfusion. Since perfusion measurements are based on the change in tissue signal intensity between the baseline and during the first pass passage of the contrast material, it is important that the MR signal strength be made insensitive to variations from other factors unrelated to the primary mechanism for signal intensity changes due to perfusion. One such variable is the magnitude of the longitudinal magnetization Mz, which is tipped into the transverse plane by the RF excitation pulse in the MR pulse sequence. After each excitation, the longitudinal magnetization is reduced and recovers magnitude at a rate determined by the T1 constant of the particular spins being imaged. If another pulse sequence is played out before the longitudinal magnetization has fully recovered, the magnitude of the acquired MR signal will be less than the signal produced by a pulse sequence which is delayed long enough to allow full recovery of the longitudinal magnetization. Moreover, if the delay time varies as a result of variations in the patient""s cardiac heart rate, the amount of longitudinal magnetization available will vary between heartbeat to beat heartbeat. This will cause fluctuations or variations in the signal intensity in the myocardial tissue independent of perfusion. It is known that the use of a saturation rf pulse with a flip angle of 90xc2x0 will set the longitudinal magnetization to zero. Thus by waiting a pre-determined and fixed time after the saturation rf pulse before imaging, the re-growth the longitudinal magnetization is dependent on the tissue spin-lattice relaxation time, T1. Since the contrast agent effects T1, the use of a saturation rf pulse will yield a signal intensity that is dependent on the concentration of the contrast material present in a region of myocardial tissue and not variations in the patient""s heart rate. The same technique is also applicable to T2 or T2* shortening agents.
Typically, perfusion imaging is a technique used to rapidly acquire images during the first pass of the contrast agent/bolus through the blood stream by using carefully optimized pulse sequence parameters. The goal of myocardial perfusion imaging is to detect and characterize any abnormal distribution of myocardial blood flow. Perfusion deficits are indicative of areas of compromised blood flow. These perfusion deficits may be transient, whereby the region of myocardial tissue is still viable and continues to receive some supply of blood, or acute where the blood flow to that region has been compromised sufficiently to render cellular damage to the myocardial tissue (i.e., myocardial infarction). Non-viable infarcted tissue undergoes cellular changes that damage the ability of the myocardial tissue or muscle to contract. Hence, regions of myocardial infarction are often characterized by having abnormal cardiac wall motion at rest. Under certain conditions where the tissue is still viable, with increased blood flow to that region, the myocardial tissue begins normal contractile motion. This type of characteristic is attributed to stunned or hibernating myocardium where the tissue is still viable but severely under perfused.
The area of cellular damage or myocardial infarction is often assessed to better determine the course of patient management. In some cases, in the periphery of the infarcted tissue, some recovery of function may be possible. However, in regions where the damage leads to micro-vascular obstruction, no recovery is possible. The use of imaging of myocardial infarcted regions allows the assessment of the extent of the cardiac injury and permits the monitoring of the patient""s response to a specific treatment regimen.
In order to assess for the presence of myocardial infarction, an inversion recovery pulse sequence is routinely employed to suppress normal myocardial tissue subsequent to the administration of the contrast bolus, which is typically between 0.1 and 0.2 mmol/kg of gadolinium contrast material. In this application, the bolus has the effect of shortening the T1 time of the blood.
During the first pass of the contrast material, under resting conditions, the infarcted region may be identified by regions of abnormal perfusion. That is to say, the infarcted zones, having very low blood perfusion would be hypo-intense relative to normal, healthy myocardium. With recirculation of the contrast material, transport of the contrast material to the site of the myocardial infarct is by the limited blood flow to the affected region or by diffusion into the extra-cellular space. Consequently, the uptake of contrast material by infarcted tissue occurs at a much slower rate than normal, healthy myocardial tissue. As the uptake of the contrast material is slow, so is the wash-out of the contrast material from the infarcted zones. This yields a phenomena whereby the infarcted region is hypo-intense during the first pass of the contrast material, reaches iso-intensity at some point in time, and at a much later or delayed time following the initial administration of contrast material, is hyper-intense relative to the normal, healthy myocardium.
Since gadolinium contrast material will concentrate in infarcted tissue, an inversion recovery magnetization preparation pulse sequence is designed with an inversion time, TI, to suppress signal from normal, healthy myocardial tissue to yield regions with high signal intensity in the infarcted zones. However, as the concentration of contrast material in the blood is still relatively high, but less than that in infarcted tissue, the ventricular blood pool will still exhibit high signal intensity relative to the infarcted tissue such that delineation of the myocardial boundaries with the ventricular blood pool cavity is difficult. That is, an MR image of blood acquired using a fast gradient echo pulse sequence, or similar technique, will display blood with a high signal intensity with respect to adjacent stationary tissue of the vessel structure. However, it is important in the diagnosing of the extent of myocardial infarction to be able to discriminate between the margins of the infarcted tissue, especially sub-endocardially, and the ventricular blood cavity.
It would therefore be desirable to have a method and apparatus that is capable of myocardial infarction detection with suppression of blood pool signal, in order for the myocardial borders to be accurately defined against a low signal intensity background of normal myocardial tissue or blood in the left ventricle.
The present invention relates to a technique for acquiring MR images to detect infarcted myocardial tissue that solves the aforementioned problems.
The invention includes the use of a blood suppression pulse used in conjunction with an inversion recovery pulse in a gated, segmented k-space gradient echo acquisition to improve the delineation of infarcted myocardium from the ventricular blood pool. The technique uses a notched inversion RF pulse with a stop-band positioned over the imaged slice. In this manner, the contrast in the imaged slice is unaffected by the blood suppression pulse. The inversion time is chosen such that blood inverted in the pass-band of the notched RF pulse, will be at or close to the null point at the time of the gradient echo read-out segment. It is expected that the concentration of the contrast material in the infarcted zones is higher than in the normal, healthy myocardium about 5-15 minutes after the administration of the contrast material. Since the concentration of the contrast material in blood is higher than that of the normal, healthy myocardium, signal intensity of the ventricular blood is higher than that of the normal myocardium. The relative concentration of the contrast material between the infarcted tissue and blood is not well defined as it is affected by several physiological factors. It is safe to assume that the concentration of contrast agent in the ventricular blood is at least as high as that in the infarcted tissue, leading to both tissues having similar T1 relaxation times and similar signal intensities.
It is thus desirable to employ a technique that suppresses signal from the ventricular blood and not from the infarcted region. A conventional slice-selective inversion pulse would null signal from both blood and the infarcted tissue. A solution to this problem requires a new approach for blood suppression while leaving the signal from the infarcted zone relatively unaffected.
The notched inversion pulse inverts the spins of blood outside of the imaged slice while not affecting the spins in the imaged slice. At some time after the applied notched inversion pulse, selected for the time needed to null signal from contrast-enhanced blood, blood from within the imaged slice would be replaced by spins from outside of the slice, thereby achieving blood suppression without affecting the signal from the infarcted zones.
In accordance with one aspect of the invention, a method of acquiring MR images with blood pool suppression includes administering a contrast agent into the blood stream of a patient, and then sometime later, applying a pulse sequence that includes a slice-selective inversion pulse and a notched inversion pulse. The slice-selective inversion pulse is designed to suppress myocardial tissue and is followed by the notched inversion pulse that is designed to suppress blood pool about the region-of-interest. MR data can then be acquired in the region-of-interest and an MR image can be reconstructed from the acquired data to assess a presence of myocardial infarction. Preferably, the longitudinal magnetization from the blood in regions outside of the imaged slice by the notched inversion rf pulse is affected in a manner such that at the time when MR data is acquired, signal from blood outside of the imaged slice that flows into the specific region-of-interest is suppressed.
In accordance with another aspect of the invention, an MR pulse sequence is disclosed having at least a primary slice-selection inversion RF pulse, a notched inversion RF pulse, and at least one imaging RF pulse. The primary slice-selection inversion RF pulse has a pass band greater in width than that of the image slice pulse. The notched inversion RF pulse is transmitted after the primary slice-selection inversion RF pulse and has a stop-band coincident with that of the primary slice-selective inversion RF pulse. A segmented k-space gradient echo imaging segment-acquisition is used in which the imaging RF pulses are separated by an inversion time TIBSP from the notched inversion RF pulse to allow excited blood from the pass-band to flow into the imaged slice region at the point when the longitudinal magnetization of the blood is close to the null point to substantially reduce the signal from blood in the imaged slice.
In accordance with yet another aspect of the invention, a computer system is disclosed for use with an MRI apparatus having a computer programmed to transmit a slice-selective inversion pulse to suppress signals from normal myocardial tissue, and then transmit a notched inversion RF pulse with an inversion time selected to invert regions outside a desired imaging slice such that signals from blood flowing into the desired imaging slice are suppressed. After the transmission of excitation RF pulses, data can then be acquired from the desired imaging slice having signal suppression from both normal myocardial tissue and blood to accentuate infarcted myocardial tissue.